Recently, a ceramic substrate which is a ceramic sintered body has been widely used as an isolated substrate comprising a circuit board. In case of a circuit board to be used for, for example, a semiconductor module provided with a semiconductor element having large electric power and large heating value, a high degree of mechanical strength, thermal conductivity, and electric isolation are required, and thus, ceramic substrates made of sintered bodies of aluminum nitride, silicon nitride, etc., are widely used.
Here, a circuit board comprises a ceramic substrate provided its one side of with a metal substrate functioning as a circuit plate on which a semiconductor is mounted, and provided on its other side with a metal substrate functioning as a heat sink connected to a radiation member, and the like. Usually, the ceramic substrate and the metal substrates are attached by a direct bonding method (DBC) or an active metal bonding method (AMB). In both bonding methods, if an excessively large waving is present on the surface of the ceramic substrate, voids are generated at the bonding boundary between the ceramic substrate and the metal substrate, the strength is lowered at the bonding boundary, and other bonding qualities are decreased. Therefore, the mechanical performance, the thermal performance, and the electrical performance of the circuit board are decreased. Then, in order to obtain a long-life circuit board having a superior property, decreasing the surface waviness of the ceramic substrate has been desired. Particularly, in recent years, the thickness of the ceramic substrate tends to be thinner to the thickness of 1.0 mm or lower in order to lower the thermal resistance of the entire circuit board, and in such a thin ceramic substrate, waving is easily generated on the surface due to deformation at the time of sintering. Therefore, there has been an increasing demand for the decrease of surface waviness.
Examples of the technology answering such a demand are disclosed in the following Patent Documents 1 and 2. Namely, Patent Document 1 discloses “a method for producing a ceramic sintered body comprising: molding a ceramic green sheet; coating a release agent containing BN powder on the surface of the green sheet by a roll coater to have 0.3 to 3 mg/cm2 of BN powder, the BN powder having an oxygen content of 3% by weight or less and an average particle diameter of 20 μm or less; laminating a plurality of green sheets; degreasing; pressing the upper and lower surfaces of the stack by setters made of BN and storing the stack in the closed container made of the same material as the setter; and sintering”, and discloses a plate-like aluminum nitride sintered body produced by this method and having a flatness of 100 μm or lower.
Further, Patent Document 2 filed in the name of the present applicant discloses “a method for producing a silicon nitride substrate comprising, laminating a plurality of green sheets with a separation material between each of the green sheets, subjecting the stack to sintering, separating the sintered stack to obtain a plurality of silicon nitride sintered bodies, and obtaining silicon nitride substrates from the silicon nitride sintered bodies, wherein the separation material is boron nitride (BN) powder having an oxygen content of 0.01 to 0.5% by weight, an average particle diameter of 4 to 20 μm, a specific surface area of 20 m2/g or less, and the BN powder is coated on the green sheet surface in a coating amount of 0.05 to 1.4 mg/cm2”, and discloses “a silicon nitride substrate mainly comprised of Si3N4, wherein a coefficient of variation Cv showing the distribution of the amount of B derived from the BN remaining on the surface of the silicon nitride substrate is 1.0 or lower, waviness Wa of the silicon nitride substrate surface is 1.5 μm or lower (with the proviso that the waviness is obtained by measuring a filtered centerline waviness using a surface roughness meter, and calculating an arithmetic average waviness Wa thereof, namely, an arithmetic average of absolute values of the deviation from the average surface height value, under the measurement conditions including an evaluation length of 30 mm, a measurement velocity of 0.3 mm/s, a cut-off value (λc) of 0.25 mm, and a cut-off value (λf) of 8.0 mm), and a relative density is 98%”.